The present invention relates generally to antenna systems, and more particularly to a reconfigurable antenna system for a satellite capable of operating in multiple orbits.
In satellite communications systems, signals are typically beamed between satellites and fixed coverage areas on Earth. With the expanding applications of satellites for many different aspects of communications, market requirements are continuously changing. Accordingly, a satellite must be capable of adapting to changes in the location of the requests for service.
There are many applications where the satellite is required to provide certain coverage beams for a primary mission and also serves as an in-orbit spare for a back-up mission providing coverage to a different set of beams. For example, satellites that provide local television channels to various designated market areas where the satellite is required to cover certain areas for a primary mission and a completely different set of market areas for a back-up mission. Thus antennas provided on satellites must be capable of reconfigurable coverages.
A reconfigurable multiple beam phased array antenna is an ideal solution to the ever changing beam coverage requirements. Beam coverage can be in the form of a number of spot beams and regional beams located over specific regions. Spot beams cover discrete and separate areas such as cities and counties. Regional beams cover larger areas such as countries. Regional beams are generated by combining a plurality of spot beams. Spot beams are generated by energizing radiating elements with selected amplitudes and phases. A reconfigurable multiple beam phased array antenna should be capable of reconfiguring the location of the beams, the size of the beams, and the power radiated in each beam.
Prior art reconfigurable multiple beam phased array antennas have uniform sized beams and employ a large number of phase shifters, which are used to steer the beams. The number of phase shifters is typically the number of elements multiplied by the number of beams, which results in a large, complex system. Further, the prior art reconfigurable multiple beam phased array antennas have limited bandwidth due to frequency scanning of the beams. The limited bandwidth causes the antenna gain and the co-channel interference to degrade.
In an attempt to address these problems, a reconfigurable multiple beam phased array antenna employing a two-dimensional stack of Rotman lenses in a low level beam forming network has been suggested as described in U.S. Pat. No. 5,936,588. However, this complex method of reconfiguring beams uses a phased array antenna with an active beamforming network. This is a very expensive solution in that the system requires a large number of amplifiers, active components and beamformers.
There is a need for a simple and efficient method of reconfiguring antenna beams among different orbital slots of a communication satellite with moderate flexibility.
It is an object of the present invention to provide a simple and efficient method for reconfiguring antenna beams among different orbital slots of a communication satellite. It is another object of the present invention to reconfigure the antenna in conjunction with body-biasing the satellite.
It is a further object of the present invention to bias the body of the satellite at one or more orbital slots for a back-up mission and have no body-bias for the primary mission while gimballing the antennas to steer the beams for the back-up mission.
A two-step method is used to reconfigure beams among different orbital slots. The satellite is biased at one or more orbital slots for a back-up mission, while it looks at the sub-satellite point for the primary mission. Body steering is achieved using pitch and roll biases.
The main reflector of the antenna is gimballed in order to steer the beams to different locations for the back-up mission. Beam size control is achieved through the use of different sized reflectors and by varying the feed size.
There are several advantages to the method of the present invention. Typically, scanning beams results in a reduction in performance. With the present invention, beam reconfiguration is possible over widely separated orbital slots due to main reflector gimballing, which reduces effective beam scanning by half compared to conventional methods and therefore, causes minimum degradation to the beam patterns.
These and other features of the present invention will be better understood with regard to the following description, appended claims, and accompanying drawings.